GENETIC DIVERSITY IN CUCUMBER (CUCUMIS-SATIVUS L) .1. A REEVALUATION OF THE US GERMPLASM COLLECTION

Genetic variation within the U.S. cucumber collection (Cucumis sativus var. sativus L. and var. hardwickii (Royle) Alef.) was assessed by ex aming the variation at 21 polymorphic isozyme loci and comparing the r esults of this investigation with a similar previous analysis of 14 lo ci. About 29% (15 of 51) of the enzyme systems examined in an initial survey were polymorphic. Seven loci (Ak-2, Ak-3, Fdp-1, Fdp-2, Mpi-1, Pep-gl and Skdh) which were not previously used to estimate genetic di versity, were assessed. On average, 1.4 loci were polymorphic per enzy me system and 2.2 alleles were present per polymorphic locus. The freq uency of polymorphisms was relatively low for Fdp-1(2) (0.01), Mpi-1(1 ) (0.03), and Skdh(1) (0.02). Principal component and cluster analyses of allelic variation at polymorphic loci separated a diverse array of 757 cucumber accessions from the U.S. National Plant Germplasm Systes m's (NPGS) collection into distinct groups by country (45 nations exam ined). All accessions of C. a, var. sativus were isozymically distinct from C. s. var. hardwickii, which were themselves dissimilar from eac h other. Data suggest that C. s. var. hardwickii is not a feral deriva tive of extant C. s. var. sativus populations. The allelic profile of C. a. var. sativus accessions originating from Burma, Thailand, Indone sia, Hong Kong, Zimbabwe and Ethiopia were distinct from the other acc essions examined. Allelic fixation has occurred at Pgd-2 in accessions from Burma, and at Ak-2 in accessions from Zimbabwe and Ethiopia. Som e of the countries examined that were in close geographic proximity (e .g., Thailand, Indonesia and Hong Kong) contained accessions with simi lar isozyme profiles. Accessions are fixed for certain alleles [e.g., Gr (1) (100%), Fdp-1(1) (100%) and Mpi-2(2) (50%) for accessions from Thailand, Indonesia and Hong Kong]. Grouping countries by continent or sub-continent (i.e., North and South American, China, Eastern Europe, Western Europe) and by numbers of accessions examined (i.e., India/Bu rma, Iran, Japan, Turkey, and remaining accessions) was used to identi fy accessions with unique aIlozymic profiles [(PIs 209064 (USA), 25748 6 (China), 188749 (Egypt), 285607 (Poland), 369717 (Yugoslavia), 35784 4 (Poland), 255936 (Netherlands), 183127 (India), 200818 (Burma), 2008 15 (Burma), 137836 (Iran), 227013 (Iran), 227235 (Iran), 451976 (Japan ), 181752 (Syria), 181874 (Syria), 169383 (Turkey), 171613 (Turkey)].